Polyamide resins have been used popularly for automobile and electric or electronic fields because of their excellent mechanical properties, heat resistance, chemical resistance, and flame retardancy. Different from the other resins, the polyamide resins are particularly excellent in long-term heat aging resistance so that they are used for parts, for example in an automobile engine room, to which heat of very high temperature is applied. The environmental temperature in the automobile engine room becomes increasingly high with a rise in the density of parts in the engine room and an increase in engine output. Accordingly, the polyamide resins are demanded to have a higher long-term heat aging resistance.
A number of technical improvements have been made to satisfy the above-described demand. Examples of them include a polyamide resin composition containing a copper halogen compound and a polyamide resin composition containing an aromatic amine or hindered phenol antioxidant. Of these, a technique for improving heat aging resistance using a mixture of a copper compound and a halogen compound is particularly excellent from the viewpoint of cost and it is used widely now.
A copper compound and a halogen compound are incorporated in a polyamide resin, for example, by preparing an aqueous solution containing the copper compound and the halogen compound and adding the resulting solution in a polymerization step, such as prior to polymerization or during polymerization of the polyamide, or adding it at any time after polymerization (blending, melt kneading, or the like with the polyamide resin). The method of adding it during a polymerization step is desired for uniformly dispersing the copper compound and the halogen compound, but appearance of intermediate products at the time when brand of these raw materials is exchanged or necessity of washing of a polymerizer may deteriorate a production efficiency. On the other hand, in the method of adding after polymerization, dispersibility becomes inferior to that in the method of adding it during a polymerization step, which leads to deterioration in the mechanical physical properties. This method has another problem that a molded or formed product of the polymer gradually absorbs water, and its color changes from white to green and therefore has poor appearance. Furthermore, extent of this color changing increases with an increase in copper concentration.
In the method of adding the aqueous solution after polymerization, methods of using various compounds in combination in order to improve the dispersibility of the copper compound and the halogen compound (refer to, for example, Patent Documents 1 and 2) are proposed.
Patent Document 1 discloses incorporating, as a lubricant, a higher fatty acid such as lauric acid, a higher fatty acid metal salt of, for example, a higher fatty acid and aluminum, a higher fatty acid amide such as ethylenebisstearylamide, or a wax such as polyethylene wax. According to the technique disclosed therein, addition of finely-ground halogen compound is proposed, based on considerations that it is not preferred to add an aqueous solution to a hygroscopic polyamide resin, or it is preferred not to incorporate water in a polyamide resin composition which is material of a molded or formed product. It is also proposed that since the halogen compound is apt to agglomerate and become solidified easily when it is added in a finely-ground form without taking any measure, a lubricant is added to the halogen compound in advance to suppress agglomeration and solidification, thereby enabling addition of it in the finely-ground form and improving the appearance.
Patent Document 2 discloses a tablet for stabilizing a polyamide obtained by mixing Stabilizer 1 (CuI, CuBr or the like) having an average particle size not greater than 2 μm, Stabilizer 2 (CuI, CuBr, KI, KBr, or the like) having a particle size ranging from 2 to 200 μm and a wax having an alkyl group with 15 or greater carbon atoms and having, in the molecular chain thereof, a functional group selected from acids, amides, esters, and allyl. This document discloses that the tablets from which powders do not drop easily by friction can be obtained by the above-described technique.
In each of these techniques, a copper compound and a halogen compound are incorporated in a polyamide without using a solvent such as water, but there is still a room for improvement in copper deposition, metal corrosion, and agglomeration of a halogen compound.
Thus, by the conventional method of adding a copper compound and a halogen compound after having polymerized a polyamide, uniform and fine dispersion of the copper compound and the halogen compound has not been achieved yet and therefore a polyamide resin composition having a satisfactory performance cannot be obtained.    [Patent Document 1] Japanese Patent Laid-Open No. Sho 50-148461    [Patent Document 2] U.S. Pat. No. 5,686,513